Method of making an electrical multilayer interconnect

ABSTRACT

A method of making an electrical multilayer interconnect in which the electrical lines can be protected by an overcoat. The method includes depositing an electrically conductive layer on a substrate, forming a base plating mask on the electrically conductive layer, plating a copper base into an opening in the base plating mask onto the electrically conductive layer, stripping the base plating mask, forming a pillar plating mask on top of the copper base, plating an electrically conductive metal pillar into an opening in the pillar plating mask onto the top of the copper base, stripping the plating mask, and stripping the electrically conductive layer below the stripped base plating mask. Additionally, a protective overcoat layer can be deposited on the exposed copper surfaces.

BACKGROUND OF THE INVENTION

Advances in integrated circuits used in high performance systems have created a demand for higher density electrical interconnections to accommodate large numbers of inputs and outputs. Copper is the preferred conductor for such high density multilayer interconnects. The drawback of copper is that it is very subject to corrosion and oxidation. To fabricate reliable multilayer copper interconnects, the copper can be protected by other materials.

The present invention is directed to an easy and economical method to fabricate an electrical multilayer interconnect having high density electrically conductive lines in a multilayer interconnect system in which the conductive lines can be protected by protective coating.

SUMMARY OF THE INVENTION

The present invention is directed to a method of making an electrical multilayer interconnect in which the electrical lines can be protected by an overcoat.

One feature of the present invention of making an electrical multilayer interconnect is depositing an electrically conductive layer on a substrate, forming a base plating mask on the electrically conductive layer, plating a copper base into an opening in the base plating mask onto the electrically conductive layer, stripping the base plating mask, forming a pillar plating mask on top of the copper base, plating an electrically conductive metal pillar into an opening in the pillar plating mask onto the top of the copper base, stripping the plating mask, and stripping the electrically conductive layer below the stripped base plating mask.

A further feature of the present invention is wherein the electrically conductive layer is a metal.

Still a further feature of the present invention is wherein the electrically conductive layer is electrolessly deposited.

Yet a further feature of the present invention is wherein the electrically conductive layer is sputter deposited.

A still further feature of the present invention is wherein the electrically conductive pillar is plated by electroless deposition.

Yet a further feature of the present invention is wherein the electrically conductive pillar is plated by electroplating.

Still a further feature of the present invention is wherein the electrically conductive metal pillar is copper, nickel, or a nickel alloy.

A still further feature of the present invention is wherein the exposed surfaces of the copper base and electrically conductive metal pillar are protected by depositing an overcoat layer.

A further feature of the present invention is wherein the overcoat layer is a Chemically inert material, such as nickel, chromium, or a nickel alloy.

A still further feature of the present invention is wherein the overcoat layer is electrolessly deposited nickel.

Further objects, features and advantages will be apparent from the following description of presently preferred embodiments of the invention, given for the purpose of disclosure and taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-13 are fragmentary elevational views in cross-section illustrating a first embodiment of a method of manufacture of an electrical multilayer interconnect in accordance with the present invention, illustrating successive stages in the manufacturing process.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention for fabricating an electrical multilayer interconnect is now described. Referring now to FIG. 1, the referenced numeral 10 generally indicates any suitable substrate. Preferably the substrate 10 includes a ceramic base 12, such as alumina ceramic or glass ceramic covered with a polyimide layer 14 having a polished surface 16. The polyimide layer 14 may have any suitable thickness, such as 10 um. The base material 12 may be of other suitable materials, such as silicon or various metallic materials.

Referring now to FIG. 2, an electrically conductive layer 24 is deposited on substrate 10. For instance layer 24 can consist of layers 18, 20 and 22 deposited on the surface 16 of the substrate 10. Preferably a sputter deposited titanium layer 18, a copper layer 20, and a titanium layer 22 sandwich is used. The bottom titanium layer 22 is used as an adhesion layer for the pre-plate Copper layer 20 and the top titanium layer 18 is used as a protective coat to prevent the copper layer 20 from oxidizing after deposition. For example only, the titanium layer 18 may be 500 angstroms thick, the copper layer 20 may be 2500 angstroms thick and the titanium layer 22 may be 500 angstroms thick. Other suitable electrically conductive layers 24 may consist of chromium/copper/chromium sandwiched layers. Alternatively, electrically conductive layer 24 can be electrolessly deposited onto substrate 10. For instance, layer 24 can comprise electrolessly deposited copper, nickel, a nickel alloy, or a combination thereof, having a thickness of 500-10,000 angstroms, wherein the preferred thickness is 1,000 angstroms.

Referring now to FIG. 3, a copper base plating mask, such as photoresist 26, is formed on electrically conductive layer 24.

Referring now to FIG. 4, the photoresist 26 can be patterned through standard lithographic methods to provide selected openings therein for pattern plating a copper base. If desired, photoresist 26 can also be patterned for plating an electrical conductor such as a reference plane, x-line or y-line. It is also noted in FIG. 4 that the top titanium layer 18 was stripped in a mild solution, such as 0.5% hydrofluoric solution, thereby forming a pattern for a copper conductor to be plated to the copper layer 20.

Referring now to FIG. 5, copper base 28, for example a reference plane, is plated to the copper layer 20 in the patterned openings formed by the photoresist 26. The copper base 28 can be plated by any suitable method, including electroplating and electroless deposition.

Referring now to FIG. 6, the photoresist 26 has been stripped as is conventional.

Referring now to FIG. 7, a pillar plating mask shown as a thick layer of photoresist 30 is formed on top of copper base 28.

Referring now to FIG. 8, the photoresist 30 is patterned as is conventional to form patterns for plating a pillar or via onto the copper base 28.

Referring now to FIG. 9, electrically conductive

metal pillar 32 is plated into an opening in photoresist 30 onto the top of copper base 28. The pillar 32 can be plated by any suitable method, including electroplating and electroless deposition. Suitable metals for pillar 32 include copper, nickel, and nickel alloys.

Referring now to FIG. 10, the photoresist 30 used as a pillar plating mask has been stripped as is conventional leaving pillar 32 securely attached to copper base 28 with the surfaces exposed.

Referring now to FIG. 11, the electrically conductive layer 24 below the stripped base plating mask 26 is stripped. It is seen that titanium 18, copper 20 and titanium 22 are stripped to disconnect connections between adjacent copper bases 28.

It has been noted that in copper/polyimide interconnects the copper which is in contact with polyimide will discolor indicating its oxidation or corrosion. This oxidation frequently results in some amount of delamination. Therefore, to enhance the reliability of the interconnect an overcoat can be provided to protect the copper from oxidation and corrosion and to enhance the adhesion of polyimide to copper. An overcoat layer may be similarly advantageous for other interconnect metals as well. Thus, if desired, the structure shown in FIG. 11 can have a protective overcoat layer deposited on the exposed surfaces, which include the exposed top surface and side walls, of the copper base 28 and metal pillar 32. Alternatively, if metal pillar 32 is not copper then the overcoat material can be deposited only on the exposed copper surfaces of copper base 28. The overcoat can be deposited by any suitable method, including immersion coating and electroless deposition. If the overcoat is immersion coated, the overcoat material can be tin, or any other material that can be immersion coated. If the overcoat is electrolessly deposited, the overcoat material can be chromium, nickel, a nickel alloy, or any other material that can be electrolessly deposited.

If desired, a light copper etch can be applied to the exposed copper surfaces prior to depositing the protective overcoat so that any copper oxide that has formed will be removed. Any suitable copper etchant can be used, such as ammonium persulfate/phosphoric acid solution. The copper etch may etch back a small amount of copper, for instance 1,000 angstroms, but this should have no significant effect on the performance of the remaining copper structure.

Referring now to FIG. 12, a protective overcoat 34, such as 5,000 angstroms thick, may be plated on all of the exposed surfaces of the metal pillar 32 and copper base 28.

Referring now to FIG. 13, if desired, the structure shown in FIG. 12 is ready for conventional polyimide deposition and polishing for planarization. In addition, more levels of conductors and reference planes can be fabricated by repeating the above-described sequence of operations. In the example given in FIG. 13, copper bases 28a can be x-lines, copper bases 28b can be y-lines, and copper base 28c is an additional reference plane with interconnecting pillars 32a, 32b, and 32c, all enclosed in a polyimide enclosure 40. Thus, the process sequence described in this invention is repeated to fabricate a high-density multilayer interconnect. Also, on top of the multilayer interconnect bonding pads 42 and barrier/adhesion layer 41 can be fabricated as is conventional to form bonding pad for component bonding to the interconnect.

A nickel alloy is described herein as suitable for electrically conductive layer 24, electrically conductive metal pillar 32, and protective overcoat 34. Acceptable compositions of these nickel alloys will be well known to those having skill in the art. By way of example, suitable nickel alloys include nickel-phosphor, nickel-tungsten, nickel-platium, and nickel-chromium.

The present invention, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While presently preferred embodiments of the invention have been described for the purpose of disclosure, numerous changes in the details of construction, and steps of the process, will be readily apparent to those skilled in the art and which are encompassed within the spirit of the present invention and the scope of the appended claims. 

What is claimed is:
 1. A method of making an electrical multilayer interconnect comprising:depositing an electrically conductive layer on a substrate, forming a base plating mask on the electrically conductive layer, plating a copper base into an opening in the base plating mask onto the electrically conductive layer, stripping the base plating mask, forming a pillar plating mask On top of the copper base, plating an electrically conductive metal pillar into an opening in the pillar plating mask onto the top of the copper base, stripping the pillar plating mask, and stripping the electrically conductive layer below the stripped base plating mask.
 2. The method of claim 1 wherein the electrically conductive layer is a metal.
 3. The method of claim 2 wherein the electrically conductive metal layer is sputter deposited.
 4. The method of claim 2 wherein the electrically conductive metal layer is electrolessly deposited.
 5. The method of claim 1 wherein the electrically conductive metal pillar is plated by electroless deposition.
 6. The method of claim 1 wherein the electrically conductive metal pillar is plated by electroplating.
 7. The method of claim 1 wherein the electrically conductive metal pillar is selected from the group consisting of copper, nickel, and a nickel alloy.
 8. The method of claim 1 further comprising the step of depositing an overcoat layer on the exposed copper surfaces
 9. The method of claim 8 wherein the overcoat layer is deposited on the exposed surfaces of the copper base and electrically conductive metal pillar.
 10. The method of claim 8 wherein the overcoat layer is electrolessly deposited.
 11. The method of claim 10 wherein the overcoat layer is an electrolessly deposited metal selected from the group consisting of chromium, nickel, and a nickel alloy.
 12. The method of claim 8 wherein the overcoat layer is immersion coated.
 13. The method of claim 12 wherein the overcoat layer is immersion coated tin.
 14. The method of claim 8 wherein prior to depositing the overcoat layer a light copper etch is applied to the exposed copper surfaces.
 15. A method of making an electrical multilayer interconnect comprising:depositing an electrically conductive metal layer on a substrate, forming a base plating mask on the metal layer, plating a copper base into an opening in the base plating mask onto the metal layer, stripping the base plating mask, forming a pillar plating mask On top of the copper base, plating a pillar selected from the group consisting copper, nickel, and a nickel alloy into an opening in the pillar plating mask onto the top of the copper base, stripping the pillar plating mask, and stripping the metal layer below the stripped base plating mask.
 16. The method of claim 15 further comprising the step of depositing an overcoat layer on the exposed copper surfaces.
 17. The method of claim 16 wherein the overcoat layer is electrolessly deposited.
 18. The method of claim 16 wherein the overcoat layer is immersion coated. 